TSR1

TSR1 family members are essential components of the signal recognition particle (SRP)-dependent pathway that translocates secretory proteins across the endoplasmic reticulum membrane (PMID:8798620, PMID:10196219). In Yarrowia lipolytica, the TSR1 gene product is a serine-rich ER transmembrane protein with an N-terminal signal peptide, and loss-of-function mutants show severely reduced synthesis of a secretory protein precursor (PMID:8798620). Mechanistically, Tsr1p sits at the interface between the SRP-ribosome complex and the ER lumen, physically associating with the ER lumenal chaperone Kar2p, with ribosomal components bound to 5.8S rRNA, and with the SRP components Sec65p and 7SL RNA, and it restores SRP stability in an SRP-mutant background (PMID:9305926). Disruption of the S. cerevisiae homologue YHC8 causes accumulation of precursors of multiple soluble secretory proteins and of a membrane protein, with the protein localizing to the ER, confirming a conserved, direct role in co-translational ER import (PMID:10196219). The mechanistic basis of the human/mammalian protein has not been characterized by direct experiment in the available corpus.

1. 1996 Medium

   Established that the TSR1 gene product is essential for SRP-dependent secretory protein translocation, answering whether the gene functions in the ER import pathway at all.

   Evidence Suppressor genetics, complementation, pulse-chase labeling and gene disruption in Yarrowia lipolytica, showing 70% reduced precursor synthesis and an ER-membrane serine-rich protein

   PMID:8798620

   Open questions at the time

   • Demonstrated only in Yarrowia lipolytica by a single lab
   • Molecular activity within the translocation reaction not resolved
   • No direct evidence in mammalian systems
2. 1997 Medium

   Defined the physical interaction network of Tsr1p, answering how it bridges the SRP-ribosome complex and the ER lumen.

   Evidence Reciprocal co-immunoprecipitation, co-fractionation, protease protection and sedimentation analysis in Y. lipolytica identifying Kar2p, ribosomal/5.8S rRNA, Sec65p and 7SL RNA associations and restored SRP stability

   PMID:9305926

   Open questions at the time

   • Interactions characterized in a single yeast species by one lab
   • Stoichiometry and direct vs. indirect nature of each contact unresolved
   • No structural model of the assembly
3. 1999 Medium

   Showed the role is conserved in S. cerevisiae and affects diverse substrates, answering whether the translocation defect is general across secretory cargo.

   Evidence Gene disruption of YHC8, pulse-chase/immunoprecipitation of multiple secretory precursors, and immunofluorescence localization to the ER

   PMID:10196219

   Open questions at the time

   • Demonstrated in yeast only; mammalian ortholog function untested
   • Mechanism of substrate recognition not established
   • Position relative to the Sec translocon machinery not defined

• Whether the human/mammalian TSR1 protein performs an analogous role in SRP-dependent ER translocation, and its molecular mechanism, remains uncharacterized by direct experiment.
• No direct experimental characterization of the human protein
• No structural or biochemical reconstitution of the proposed activity
• Conservation of the interaction network in mammals untested